Watch: Rebecca Black performs 'Friday' with Katy Perry

Katy introduced the 14-year-old as "the infamous Rebecca Black" (LOL) as she made her surprise appearance at the gig, then the teenager dubbed "the worst singer in the world" joined Katy in a duet rendition of her horror hit song 'Friday'.

"Dammit! I love the Internet," Katy shouted as her young friend left the stage.